1. Field of the Invention
This invention relates to a reflection-preventive film for optical parts of synthetic resin.
2. Related Art Statement
Recently, synthetic resins having a light weight and which are easy to work with have frequently been used as a material for optical elements or parts such as lenses and the like instead of inorganic glass. However, the optical parts made from the synthetic resin are highly reflective and inorganic glass is soft, so that there is a problem that the surface is apt to be scratched. Therefore, it is necessary that a reflection-preventive film simultaneously serving as a curing protection be formed on the surface of the optical part made of synthetic resin.
The reflection-preventive film is usually formed by a vacuum deposition process. In the case of an inorganic glass substrate, a durable deposited film is obtained by heating the substrate at 200.degree.-400.degree. C. However, when the synthetic resin is heated at 200.degree.-400.degree. C., it is degraded and decomposed, so that the formation of the deposited film is impossible through substrate heating as with the inorganic glass.
Therefore, as disclosed, for example, in Japanese Patent laid open No. 60-225,101, the formation of the reflection-preventive film onto the surface of the synthetic resin substrate has hitherto been conducted without the heating of the substrate. This reflection-preventive film formed on the surface of the synthetic resin substrate has, for example, a five layer structure obtained by alternately and repeatedly laminating a SiO.sub.2 layer and a Ta.sub.2 O.sub.5 layer, wherein the SiO.sub.2 layer is first deposited on the substrate surface by a resistor heating process of the SiO, and the formation of Ta.sub.2 O.sub.5 layer through the resistor heating process or an electron gun process and the formation of a Si.sub.2 layer through the resistor heating process or the electron gun process are repeated. In such a reflection-preventive film, the improvement of the bonding property to the substrate and the reduction of the spectral reflectance are planned.
In the above conventional reflection-preventive film, however, micro cracks occur after repeating a thermal shock test 10 times according to a temperature cycle of (room temperature (20.degree.-25.degree. C.).fwdarw.-30.degree. C., 60 minutes.fwdarw.room temperature for 30 minutes.fwdarw.80.degree. C. for 60 minutes.fwdarw.room temperature). Furthermore, since Ta.sub.2 O.sub.5 had a high melting point, when the vacuum deposition is carried out by the electron gun process, the radiation heat is large, and consequently the substrate made from acrylic resin or the like is thermally deformed as in the case of continuous vacuum deposition to degrade the optical accuracy. Moreover, Ta.sub.2 O.sub.5 is liable to cause splashing (so-called flying, resulting in the adhesion of Ta.sub.2 O.sub.5 in granular form) to produce a surface defect, and hence there is a problem in the appearance performance.